Biomedical Engineering Reference
In-Depth Information
5
Future Trends
CHAPTER
5.1 CHALLENGES
There are limitations that challenge energy scavenging at small scales
such as the energy generation efficiency, energy density, rectification,
energy storage and management, manufacturing, longevity, and pack-
aging. A summary of these challenges is discussed to highlight some of
the difficulties that need to be resolved in the near future for energy
harvesting to be a practical energy source for portable, embedded, or
implantable applications.
Energy generation from inertial approaches at smaller scales faces
limitations due to the reduced mass sizes because power is directly pro-
portional to mass. For example, a proof mass with a volume of 1 mm
3
has a maximum available power of 1
µ
W (volumetric power density of
1 mW/cm
3
) for a generator attached to a walking person. Yet, only a
few reported devices are close to that figure of the volumetric power
density. Hence, generator size, placement, and power requirements
must be evaluated. Parallel-plate electrostatic generators need to
have air
m and hundreds of micro-
meters in order to generate power comparable to electromagnetic or
piezoelectric devices, as estimated by Roundy (2005). This displace-
ment range severely challenges the implementation and stability of
electrostatic generators. Electrical generators usually are more efficient
with increasing sizes, this means that efficiency does not scale well with
smaller dimensions. Furthermore Roundy (2005) showed that electro-
magnetic, piezoelectric, and electrostatic generation have the possibility
of high coupling coefficients (0.6
0.8). Therefore, the main challenge
is to design miniature energy harvesters with efficiencies as high as the
models predict. In addition, trying to recreate high efficiencies at smal-
ler scales is a difficult task. Finding materials with the same properties
as the bulk materials with tight tolerances and same reliability as the
larger counterparts while being cost competitive is also difficult.
Rectification is still the biggest obstacle because the threshold voltages
represent a large fraction of the low voltages produced, and this
gap displacements between 0.5
µ
